In recent years, the range of applications for magnetic recording apparatuses such as magnetic disk drives, flexible disk drives and magnetic tape drives has been markedly increased and the importance of such apparatuses has also increased. Also, the recording density of the magnetic recording media used in such apparatuses is being largely increased. In particular, a steeper increase in areal recording density followed the introduction of an MR head and a PRML technique. Since the introduction of a GMR head and a TuMR head in recent years, the recording density has been increasing at a pace of about 30-40% per year.
Under these circumstances, there is demand for achieving a further increase in recording density with respect to magnetic recording media in the future and, hence, demand for achieving a higher coercive force, a higher signal-to-noise ratio (S/N ratio) and a higher resolution of a magnetic recording layer. In a longitudinal magnetic recording system widely used heretofore, the self-demagnetization of recording magnetic domains, i.e., the action of each of an adjacent pair of recording magnetic domains in a magnetization transition region weakening the magnetization of the other, becomes dominant with increases in linear recording density. There is a need to increase the magnetic shape anisotropy in a magnetic recording layer by continually reducing the thickness of the magnetic recording layer in order to avoid the self-demagnetization.
On the other hand, as the film thickness of a magnetic recording layer is reduced, the magnitude of an energy barrier for maintaining magnetic domains and the magnitude of thermal energy become so close in level to each other that a phenomenon in which a recorded amount of magnetization is relaxed under the influence of temperature (heat fluctuation phenomenon) is not negligible. This is said to be a determinant of the linear recording density.
In such circumstances, an anti-ferromagnetic coupling (AFC) medium has recently been proposed as a technical device to meet the demand for improving the linear recording density in the longitudinal magnetic recording system, and efforts are being made to avoid the thermal magnetization relaxation problem with longitudinal magnetic recording.
Perpendicular magnetic recording techniques are attracting attention as a promising technique for achieving a further increase in areal recording density. While a medium is magnetized in a direction along the surface of the medium in the conventional longitudinal magnetic recording system, a perpendicular magnetic recording system is characterized by magnetization in a direction perpendicular to the medium surface. Perpendicular magnetic recording is thought to be a way of avoiding the influence of self-demagnetization which is a hindrance to achievement of a higher linear recording density in the longitudinal magnetic recording system, and to be more suitable for recording at a higher density. Also, perpendicular magnetic recording is thought to be comparatively unsusceptible to thermal magnetization relaxation, which is the problem with longitudinal magnetic recording, because a certain magnetic layer thickness can be maintained in the case of perpendicular magnetic recording.
In ordinary cases, a perpendicular magnetic recording medium has a seed layer, an intermediate layer, a magnetic recording layer and an overcoat formed in this order on a nonmagnetic substrate. Also, in many cases, a lubricating layer is applied on the surface after film forming of the overcoat. Also, a magnetic film called a soft-magnetic under layer is ordinarily provided under the seed layer. The intermediate layer is formed for the purpose of further improving the characteristics of the magnetic recording layer. The seed layer has a function of aligning crystals in the magnetic recording layer and also of controlling the shape of magnetic grains.
It is important to control a magnetic exchange interaction between magnetic crystal grains of the magnetic recording layer so as to manufacture a perpendicular magnetic recording medium having excellent characteristics. Since a noise component increases when the exchange interaction is too strong, the recording and reproducing characteristics deteriorate. In contrast, when the exchange interaction is too weak, heat fluctuation characteristics deteriorate. In a conventionally used granular structure, ferromagnetic Co alloy crystals are surrounded with crystal grain boundaries made of a nonmagnetic oxide or nitride, and the exchange interaction between magnetic crystal grains is controlled by the grain boundaries.
In the granular structure, it is difficult to control since the grain boundary width becomes ununiform, resulting in ununiform exchange interaction. Therefore, the exchange interaction on the film in-plane direction is uniformized by forming a magnetic recording layer including no grain boundaries made of an oxide or nitride on the magnetic recording layer having a granular structure, and thus the recording and reproducing characteristics are improved (Patent Document 1). However, because of surface unevenness caused by lamination of a seed layer, an intermediate layer and a granular magnetic recording layer, the magnetic recording layer including no grain boundaries made of an oxide or nitride is not a completely uniform continuous film, and also a layer in which each crystal grain is separated exists, resulting in ununiform exchange interaction.
It is necessary to obtain a perpendicular magnetic recording medium having excellent recording and reproducing characteristics, which can uniformize the exchange interaction in the film in-plane direction of a magnetic recording layer so as to improve the recording and reproducing characteristics in future. It has been required to obtain perpendicular magnetic recording medium which can solve such problems and also can be easily manufactured.
Patent Document 1:
Japanese Unexamined Patent Application, First Publication No. 2004-310910